The present invention concerns a process for selective hydroisomerisation of long (more than 10 carbon atoms), linear and/or slightly branched paraffins, in particular for high yield conversion of feeds with high pour points to at least one cut with a low pour point and a high viscosity index.
High quality lubricants are of fundamental importance for the efficient operation of modern machines, cars and trucks. However, the quantity of paraffins which directly originate from untreated crude and which have the properties to constitute good lubricants is very low when compared with the increasing demand in this sector.
Heavy oil fractions containing large amounts of linear or slightly branched paraffins must be treated to obtain good quality lubricant stock in the best possible yields. A dewaxing operation is used which is intended to eliminate the linear or very slightly branched paraffins from feeds which are then used as lubricant stock or as kerosine or jet fuel.
The linear or very slightly branched high molecular weight paraffins which are present in the oils, kerosine or jet fuel produce high pour points and thus lead to coagulation at low temperatures. To reduce the pour points, these linear or very slightly branched paraffins must be completely or partially eliminated.
The dewaxing operation can be effected by extraction using solvents such as propane or methyl ethyl ketone, using propane or methyl ethyl ketone (MEK) dewaxing. However, such techniques are costly, long and not always easy to carry out.
Catalytic dewaxing, as opposed to solvent dewaxing, is more economical and can produce products with the desired physico-chemical properties. This is achieved by selective cracking of the longest linear paraffin chains which leads to the formation of lower molecular weight compounds, a portion of which can be eliminated by distillation.
Because of their form-selectivity, zeolites are among the most widely used dewaxing catalysts. The idea that anticipated their use is that zeolite structures exist in which the pore openings are such that they allow long linear or very slightly branched paraffins to enter their microporosity but branched paraffins, naphthenes and aromatics are excluded. This phenomenon thus leads to selective cracking of linear or very slightly branched paraffins.
Zeolite based catalysts with intermediate pores such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and ZSM-38 have been described for use in catalytic dewaxing by cracking.
Processes using those zeolites can achieve dewaxing by cracking of feeds containing quantities of less than 50 weight % of linear or very slightly branched paraffins. However, with feeds containing higher quantities of these compounds, cracking leads to the formation of large quantities of lower molecular weight compounds such as butane, propane, ethane and methane, which considerably reduces the yield of the desired products.
In order to overcome these problems, we have concentrated our research on the development of catalysts (preferably non ZSM) which could bring about isomerisation of these compounds.
A number of patents exist in this field, for example International patent application WO 92/01657 which describes and claims a process for dewaxing feeds with isomerisation in the presence of a group VIII metal at a hydrogen pressure in the range 100 KPa to 21000 KPa and using a catalyst with a pore opening in the range 0.48 nm to 0.71 nm and in which the crystallite size is less than 0.5 .mu.m. The catalyst leads to improved performances as regards yield with respect to prior art catalysts.